ihear-
Yes, women are patur from mitzvas aseh shehazman grama, and are certainly patur from tefillin. But they are chayav in tefilah. How many times a days is a machlokes rishonim, but most mainstream achronim (including the m’b) hold that women are chayav at least in shachris and mincha. Assuming women are chayav in shachris I don’t see why they wouldn’t be obligated in zman tefillah as well. Another point to consider is that if a woman wants to say birchos krias shma (which she is not required to), according to nearly all achronim, it would be assur to say it after zman tefillah.